WASHINGTON — President Obama is weighing a military strike against Syria that would be of limited scope and duration, designed to serve as punishment for Syria’s use of chemical weapons and as a deterrent, while keeping the United States out of deeper involvement in that country’s civil war, according to senior administration officials.

Such an attack would probably last two days or less and involve sea-launched cruise missiles — or, possibly, long-range bombers — striking military targets not directly related to Syria’s chemical-weapons arsenal. The timing would depend on three factors: completion of an intelligence report assessing the Syrian government’s culpability in last week’s alleged chemical attack; ongoing consultation with allies and Congress; and determination of a justification under international law.

“We’re actively looking at the various legal angles that would inform a decision,” said an official who spoke about the presidential deliberations on the condition of anonymity. Missile-armed U.S. warships are already in the Mediterranean.

As the administration moved rapidly toward a decision, Secretary of State John Kerry said the use of chemical weapons in an attack Wednesday against opposition strongholds on the eastern outskirts of Damascus is now “undeniable.”

Evidence being gathered by U.N. experts in Syria was important, Kerry said, but not necessary to prove what is already “grounded in facts, informed by conscience and guided by common sense.”

The team of United Nations weapons investigators Monday visited one of three rebel-held suburbs where the alleged attack took place, after first being forced to withdraw when their vehicles came under sniper fire.

The Syrian government, which along with Russia has suggested the rebels were responsible for the chemical attack, agreed to the U.N. inspection over the weekend.

Videos and statements by witnesses and relief organizations such as Doctors Without Borders have proved an attack occurred, Kerry said. The U.S. intelligence report is to be released this week.

Among the factors, officials said, are that only the government is known to possess chemical weapons and the rockets to deliver them, and its continuing control of chemical stocks has been closely monitored by U.S. intelligence.

Kerry said Syrian forces had engaged in a “cynical attempt to cover up” their actions, not only by delaying the arrival of the U.N. team but by shelling the affected area continually. Any strike would probably await the departure of the U.N. inspectors from Syria.

Kerry’s statement, which he read to reporters in the State Department briefing room without taking questions, was part of an escalating administration drumbeat, which is likely to include a public statement by Obama in coming days.

Officials said the public warnings are designed partly to wring any possibility of cooperation out of Russia — or an unlikely admission from the government of Syrian President Bashar Assad — before Obama makes his decision.

“Make no mistake: President Obama believes there must be accountability for those who would use the world’s most heinous weapons against the world’s most vulnerable people. Nothing today is more serious, and nothing is receiving more serious scrutiny,” Kerry said.

He and other officials drew a sharp distinction between U.S. action related to a violation of international law by what they called Assad’s “massive” use of chemical weapons and any direct military role in the Syrian conflict, now in its third year.

Obama and other officials have said repeatedly that no U.S. troops would be sent to Syria. But despite Obama’s year-old threat of an unspecified U.S. response if Assad crossed a “red line” by using chemical weapons, even a limited military engagement seemed unlikely before Wednesday’s attack.

“Potential targets include high-value regime air defense, air, ground, missile, and naval forces as well as the supporting military facilities and command nodes,” Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said in a June letter to Congress. “Stand-off air and missile systems could be used to strike hundreds of targets at a tempo of our choosing.”

Although Dempsey, who has questioned the wisdom of direct military involvement in Syria, said that such an operation would require “hundreds” of ships and aircraft and potentially cost “in the billions,” the action s being contemplated would be far smaller and designed more to send a message than cripple Assad’s military.

Syrian chemical-weapons storage areas, which are widely dispersed, are seen as unlikely targets.

House Speaker John Boehner, R-Ohio, said he had been in touch with the White House. In a statement, he echoed concerns expressed by lawmakers from both parties that the administration further consults Congress before taking action.

Authorization for the use of force against another nation normally comes only from the U.N. Security Council — where Russia and China have vetoed previous resolutions against Assad — or in a NATO operation similar to the one launched in the former Yugoslavia in 1999, without a U.N. mandate.

Britain, France and Turkey have said they would support action if the use of chemical weapons is confirmed, but a clear case is also likely to make approval easier for allies such as Germany, which disputed NATO’s 2011 operation in Libya despite a U.N. resolution.